1. Field of the Invention
The present invention relates to an active clamp forward converter, and more particularly, to an active clamp forward filter having low levels of switching loss and conductance loss.
2. Description of the Related Art
FIG. 9 is a circuit drawing showing an example of the prior art of a single-transistor, forward active clamp circuit disclosed in Japanese Unexamined Utility Model Application, First Publication No. 4-72882. This circuit is equipped with a transformer 17 in which a first end of a primary coil is connected to the positive terminal of a direct current power supply 1 via an inductor 8, and a second end of a primary coil is connected to the negative terminal of a direct current power supply 1 via a switching element 2.
A capacitor 9 and a switching element 5 are additionally connected in series between the positive terminal of direct current power supply 1 and the second end of the primary coil of the transformer 17. In addition, a diode 3 and a capacitor 4 are connected in parallel to the switching element 2, and a diode 6 and a capacitor 7 are connected in parallel to the switching element 5.
The anode side of a diode 18 is connected to a first end of the secondary coil of the transformer 17, the anode side of a diode 19 and one end of a choke coil 20 are connected to a second end of the secondary coil of the transformer 17, the cathode side of diode 18 and the cathode side of the diode 19 are connected to the positive side of an output connector, and the other end of the choke coil 20 is connected to the negative side of an output terminal. In addition, an output capacitor (smoothing capacitor) 21 is connected between the output terminals to which a load 22 is connected.
The following provides an explanation of the operation of the example of the prior art of FIG. 9.
When switching the element 2 is controlled to on, an input voltage Vin of the direct current power supply 1 is applied to the inductor 8 and the primary coil of the transformer 17, and a current rises from the inductor 8 towards the primary coil of the transformer 17 resulting in accumulation of excitation energy.
When the switching element 2 is controlled to off after a fixed amount of time, the current is maintained in the same direction by the accumulated excitation energy. Consequently, the capacitor 7 is discharged simultaneous to charging of the capacitor 4, and the diode 6 takes on a forward direction bias and is turned on causing zero voltage to be held between the terminals of the switching element 5.
During this time, the switching element 5 is controlled to on and zero voltage switching is performed.
Although the current from the inductor 8 towards the primary coil of the transformer 17 charges the capacitor 4 and the capacitor 9, this current gradually decreases and finally inverts caused by resonance phenomena due to the inductance of the inductor 8 and transformer 17 and the capacitance of the capacitor 9.
Subsequently, although the switching element 5 is controlled to off, the current from the primary coil of the transformer 17 towards the inductor 8 is maintained, and together with charging the capacitor 7, charges the capacitor 4 to generate a forward direction bias in the diode 3 causing a zero voltage to be held between the terminals of the switching element 2.
During this time, the switching element 2 is controlled to on, zero voltage switching of the main current is performed, and the voltage Vin of the direct current power supply 1 is applied to the inductor 8 and the primary coil of the transformer 17.
As a result of repeating the above operation, the current flowing to the primary coil of the transformer 17 is controlled by zero voltage switching, and the voltage induced in the secondary coil is supplied to the load 22 after being rectified by the diodes 18 and 19 and smoothened by the choke coil 20 and the output capacitor 21.
As has been described above, in this active clamp circuits, switching loss is attempted to be reduced by switching the switching element on with the zero voltage between terminals, and when off, delaying the rise of the voltage by the capacitors connected in parallel.
Here, the voltage Vin of the direct current power supply 1 is applied to the inductor 8 and the primary coil of the transformer 17 when switching the element 2 is on, while a charging voltage VcO of the capacitor 9 is applied in the reverse direction when the switching element 2 is off. However, since the time product of the applied voltage when the switching element 2 is on and off is 0 based on the conditions of magnetic flux equilibrium, the following equation is valid when the on duty factor is taken to be D:
Vinxc2x7D=VcOxc2x7(1xe2x88x92D)
Thus, the charging voltage VcO of the capacitor 9 becomes as follows:
VcO=Vinxc2x7D/(1xe2x88x92D)xe2x80x83xe2x80x83(1)
In addition, a maximum voltage VswO applied to the switching element 2 or 5 becomes as follows:
VswO=Vin+VcO=Vin/(1xe2x88x92D)xe2x80x83xe2x80x83(2)
As described above, the switching loss is reduced by a zero voltage switching or a zero current switching in the active clamp circuits. Though, in order to additionally reduce the loss caused by the on resistance of the FET (field effect transistor) used for switching element 2, it is preferable to increase the windings ratio of the primary and secondary coils of the transformer 17, decrease the current flowing to the switching element 2, and set the ratio of the maximum time at which the switching element 2 is switched on to the switching cycle, namely a maximum on duty factor Dmax, to 0.5 or more.
However, in the active clamp circuit of the prior art shown in FIG. 9, as shown in equations (1) and (2), as the on duty factor D increases, the charging voltage VcO of the capacitor 9 or the maximum applied voltage VswO of the switching element increases.
For example, if the voltage Vin from the direct current power supply 1 is taken to be 360 V, even if the the maximum on duty factor Dmax is 0.6, the voltage VcO applied to the capacitor 9 becomes 1.5 Vin=540 V in the case the on pulse has widened to the maximum on time during a sudden change in output current. In addition, the maximum voltage VswO applied to the switching element 2 ends up becoming Vin+VcO=900 V.
Consequently, the problem was encountered in which the maximum on duty factor Dmax ends up being restricted by the withstand voltage of the switching element or capacitor. In addition, if the maximum on duty factor Dmax is increased, the FET having a high withstand voltage is required for use as the switching element 2. In general, as the withstand voltage of the FET becomes higher, the on resistance of the FET also increases. Consequently, there was the problem of the conductance loss when the switching element 2 is on conversely increasing.
In addition, there was also the problem with respect to capacitor 9 in which, as the rated voltage becomes higher, the capacitor having a larger external shape is required.
The object of the present invention is to improve on these problems by providing an active clamp forward converter that reduces the maximum voltage applied to a switching element as well as the charging voltage of a capacitor, and allows the use of the switching element and a capacitor having lower withstand voltages, resulting in low on loss of the switching element and enabling the size of the capacitor to be made smaller as well as a wide control range for the on duty factor.
In order to solve the above problems, the active clamp forward converter as claimed in the present invention is equipped with:
a transformer having a primary coil of which one end is connected to a first contact, an inductor connected between the other end of the primary coil of this transformer and a second contact, a first switching element connected between the positive terminal of a direct current power supply and the second contact, a second switching element connected between the first contact and the negative terminal of the direct current power supply, a third switching element and a first capacitor connected in series between the positive terminal of the direct current power supply and the first contact, a second capacitor and a fourth switching element connected in series between the second contact and the negative terminal of the direct current power supply, first through fourth diodes respectively connected in parallel to each of the first through fourth switching elements, third through sixth capacitors respectively connected in parallel to each of the first through fourth switching elements, and a rectifying circuit connected to the secondary coil of the transformer; wherein, a pair of the first and second switching elements and a pair of the third and fourth switching elements are alternately controlled on and off sandwiched about a period during which they are both controlled to off.
As a result of adopting a constitution like that described above, the maximum voltage applied to each switching element can be reduced to xc2xd of that of the example of the prior art. In addition, when the maximum on duty factor is taken to be, for example, 0.6, the charging voltage of a clamp capacitor can be reduced to ⅓ that of the example of the prior art.
In addition, the active clamp forward converter as claimed in the present invention is characterized such that:
the inductor is substituted at the leakage inductance of the transformer, each of the first through fourth switching elements is composed of first through fourth FETs, each of the first through fourth diodes is composed of respective parasitic diodes of the first through fourth FETs, and each of the third through sixth capacitors is composed of the respective parasitic capacitance of the first through fourth FETs.
In addition, it is preferable that the above rectifying circuit is a half-wave rectifying or a full-wave circuit rectifying circuit which uses a diode for the rectifier.
In addition, it is preferable that the above rectifying circuit is a half-wave rectifying circuit or a full-wave rectifying circuit which uses an FET for the rectifier.